Thursday’s magnificent Chris Knox benefit show at (Le) Poisson Rouge is one that will give music fans a lot to ponder for years to come — along with raft of great memories.

When I wrote yesterday that I wished Jeff Mangum had not done an encore to his spectacular mid-show set, I wasn’t criticizing him or his performance. After the encore was over, I was as thrilled as anyone in the crowd to have heard him do yet another song. But before he started “Engine,” I was fearful that the perfect moment he created in his four-song set might somehow be damaged.

The Neutral Milk Hotel frontman seemed so happy to be playing in public to such an adoring audience that his return for an encore seemed natural rather than forced. I asked the show’s organizer, Ben Goldberg, about how it all happened organically.

Benefit organizer Ben Goldberg.

“It’s funny, I fully wasn’t expecting Jeff to play again once he got offstage,” Ben tells Will You Miss Me When I’m Gone? “He seemed both elated and relieved it was over.

“But then he heard people screaming and was like, ‘Should I do Engine? You think I should do Engine?’ So, it kind of felt like a true encore – not planned.

“In fact, Dimmer [the New Zealand band that played next] was already coming out to set up. I had to stop them so he could play!

“I know what you mean about the encore being a bit of a tag-on, but I don’t know…everyone singing along like they did…it seemed special in and of itself.”

The experience clearly blew away Ben — just like it did the rest of the crowd. “It’s terrible to be the organizer of a big show like that and feel completely emotionally drained halfway through!”

If you want to see photos and video of Jeff Mangum‘s long-awaited return as a spotlight performer for the first time in a decade (despite claims to the contrary, he has performed in public during that time at least once, playing one song and helping out on the Elephant 6 tour in 2008), you’ll have to look elsewhere.

Yes, Will You Miss Me When I’m Gone? was at Manhattan’s (Le) Poisson Rouge last night when the reclusive Neutral Milk Hotel frontman performed at three-song set with a one-song encore as part of a sold-out Chris Knox benefit. I got inside with a 3.2-megapixel camera-equipped BlackBerry. But surreptitious photography and videography just didn’t seem to be in the spirit of things last night.

Ben Goldberg, who organized and ran last night’s benefit and amazingly kept it running on time, made things pretty clear:

Do not photograph the bands while they play. Do not film the bands while they play. We’ve turned down some pretty incredible offers to record this for various outlets so that you can enjoy the show unencumbered, so – hey – don’t be a dick. Just soak it in, let the glory of the moment wash over you, and then spend the rest of your life reminiscing at how great it was that you are alive and were there.

And soak it in I did! But, as expected, there were more than a few dicks in the crowd last night. Sad, but a fact of life. So in about 30 seconds, Google will point you to sites with grainy photos and videos. Yes, I’ve looked at them. How could I not? But I can’t in good conscience promote them.

It was clear that many of the people who paid $75 apiece to help Chris, an influential New Zealand indie rocker who suffered multiple strokes last year, were there just to hear Jeff. While the Louisiana native never really completely dropped out of sight, he has been a man of mystery since disbanding NMH after a tour to support its final album, In the Aeroplane Over the Sea, in 1998.

Jeff was greeted with ear-splitting applause. He appeared to be genuinely gratified by the reception as he came onstage with no ceremony, set up two guitars, and sat in a chair and prepared to play. Someone in the audience yelled out ‘We missed you,” to which he responded, “I missed you too.” One concertgoer positioned close to the stage reports he added a tiny coda to that remark, saying just under his breath, “Don’t think that I haven’t.”

He launched into his set with “Oh Comely.” His voice sounded just a bit weak at first, but then it became clear he hadn’t quite adjusted to the room and the sound setup. It was a revelation to see that his distinctive vocal sound stems from the fact that he didn’t use a vocal microphone, but rather just sang loudly enough for his voice to be picked up by the mic set up for his acoustic guitar. The strain of singing so loudly, coupled with his deliberately nasal delivery, makes Jeff’s singing so memorable and touching.

The crowd went wild as he continued his set with “A Baby for Pree” and “Two Headed Boy Pt. 2.″ Then with a quick “This is my last song,” Jeff launched into the title song of NMH’s final album, “In the Aeroplane Over the Sea.” Jeff is one of the most poetic lyricists in rock music, and “Aeroplane” may be the most beautifully sad song he’s released. Many of us were in tears as he sang the lines: “And one day we will die/And our ashes will fly from the aeroplane over the sea/But for now we are young/Let us lay in the sun/And count every beautiful thing we can see.”

It was hard to imagine he could top that rendition. It was as close to perfect as live music should be. The crowd begged for more. And I hoped against hope that he’d stay in the green room, ignoring calls for an encore. (After all, this concert was a showcase-format gig, and artists don’t normally take encores.) I wanted more, but I really wanted to be left wanting. The prospect of an encore loomed like a big buzzkill for the mood Jeff’s set created.

But it was impossible for him to resist. He returned to more applause to play “Engine” — asking us to sing along — before picking up his guitars and gig bags and walking off with a huge smile on his face. I can’t complain. It was another great moment. And the people who were there just to hear Jeff wanted to hear anything. As one young fan in a Led Zeppelin T-shirt told me, “I’d listen to him string his guitar, man. I’d listen to him gargle!” And Jeff did far better than that last night.

Unlike some of last night’s performers — Kyp Malone (TV on the Radio/Rain Machine), and Georgia Hubley, Ira Kaplan and James McNew (Yo La Tengo) — Jeff didn’t hang at the side of the stage to watch other bands. But he didn’t disappear after his set. He returned to the music room, with his wife, documentary filmmaker Astra Taylor, to greet some friends and happily shake fans’ hands. He looked happy and peaceful.

The year may be far from over, but Will You Miss Me When I’m Gone? has a feeling that the Chris Knox benefit at Manhattan club (Le) Poisson Rouge tonight is very likely to be the highlight of the year for those lucky enough to have gotten in.

We may have lagged a bit it posting while we search for new funding sources to keep WYMMWIG? going, but that hasn’t kept us away from the clubs and concert halls of New York and environs. And, with a bit of luck, we’ll be back with some recent updates tomorrow.

For now, you’ll have to settle for this.

For starters, you might ask, who’s Chris Knox and why does he need a benefit?

Well, unless you’ve been living under a rock, you can probably guess the answer to the second half of the question. Chris Knox is a musician, and, like most committed, full-time musicians, he doesn’t have adequate health care. (Not to be too grim about it, but the rock world has lost way too many of its best to the lack of proper health care — think Jay Bennett, for instance.)

The reclusive Jeff Mangum of Neutral Milk Hotel is making a very, very rare appearance at the Chris Knox benefit.

Now, back to the first part of the question. Knox, 57, is a New Zealand rocker who with Alex Bathgate formed Tall Dwarfs, a pioneer of the lo-fi rock movement. He had a series of strokes last year, and now his musical descendants are lining up to pay him back for his tremendous influence by raising money to pay his medical bills.

Those musical descendants make up a list of indie rock’s’ best and brightest — and most reclusive.

The scheduled appearance of Jeff Mangum, the brains behind Neutral Milk Hotel, is stirring the most interest. He’s been rather reclusive for the last 10 years, but is slated to play a short set tonight.

And then there’s the rest of the list (and organizer Ben Goldberg of BaDaBing Records says the lineup has been changing by the day), which includes: Yo La Tengo, the Magnetic Fields’ Claudia Gonson (can Stephin Merritt stay away?), TV on the Radio’s Kyp Malone, Portastatic, The Clean, Sharon Van Etten, David Kilgour and who knows how many more.

Goldberg won’t even think about giving out a set list, so if you’re going, you need to get there early and plan to stay late. And don’t expect to see the usual host of photos on WYMMWIG? tomorrow, because all cameras are banned — there won’t even be a house photographer! Given how tightly this thing has been run, I pity the first jerk who’s caught taking photos during the show!